1. Field of the Invention
The present invention relates to an X-ray detecting apparatus, in particular, a detector for determining a type of material of an object, through which X-rays are passed.
2. Related Art
To determine the type of material of an object through which X-rays are passed, there are known detector arrangements consisting of a plurality of detectors, mainly arranged in pairs. Each pair of detectors consists of two radiation detectors, which are arranged in succession and are penetrated in succession by X-ray quanta of a radiation source, with the front detector having a lower absorption than the rear detector, especially at higher energy levels; low energy X-ray quanta are absorbed almost completely in the low energy detector positioned in front. Higher energy X-ray quanta pass through the low energy detector with almost no interaction and are absorbed in the high-energy detector at the rear. The front detector may have a lower thickness than the rear detector. In addition, the two detectors may also have different chemical compositions and densities.
With such an arrangement, a separation of X-rays into individual energy ranges is achieved, so it is possible to determine the material type of the components of the object through which the X-rays are passed. The radiation detectors consist, for example, of solid-state scintillators in combination with semiconductor photodiodes. These solid-state luminescent materials (solid-state scintillators) convert X-rays into visible radiation, for example, which is then converted to a current signal by photodiodes. The current signal is proportional to the intensity of the X-rays absorbed.
Such an arrangement for detecting X-rays is disclosed in U.S. Pat. No. 4,626,688, where the individual energy ranges of the X-rays penetrate through an object having components and are attenuated. Polycrystalline phosphors have been proposed as detector materials, consisting of elements with atomic numbers in the range of 39 to 57 in the case of low energy detectors, and elements with atomic numbers in the range of 56 to 83 for high-energy detectors. The thickness of individual detectors at which good scintillation is guaranteed is determined as a function of these atomic numbers. A filter material is placed between the two detectors to achieve better separation of the two energy ranges.
One disadvantage of this arrangement is that it is very difficult to produce these phosphors with a homogeneous mass distribution in comparison with known single crystals having a homogeneous thickness. In addition, these phosphors in comparison with single crystals often have a reduced efficiency with respect to conversion of X-rays to light. This is due to the fact that some of the scintillation light generated is dispersed and/or absorbed on the phosphor particles within the phosphor layer. Consequently, phosphor layers supply a signal having a smaller signal-to-noise ratio than the signal of many single crystal scintillators.
Another disadvantage is based on the fact that with detector materials having an atomic number of approximately 50, the K absorption edge of the corresponding element in the range of 30 keV or more. If the low energy detector contains a material of such an element, the unwanted interaction of high energy X-ray quanta with energies above 50 keV in the low energy detector increases, with the main portion of the X-ray spectrum which is absorbed in the low energy detector being shifted toward the higher X-ray spectrum, thus reducing the quality of the separation of the two energy ranges.
German Patent 44 02 258 A1 describes a detector for detection of high-energy radiation, consisting of hot-pressed luminescent material and a photodiode or a photomultiplier. This luminescent material, which is also known as pressed ceramic, has good scintillation properties. The high energy X-rays are absorbed in the luminescent material, emitting as a result of this absorption visible light which is detected by the photodiode (photosensitive element). The luminescent material disclosed here is based on elements of a rare earth oxysulfide and has a low persistence.
German Patent 44 27 021 A1 discloses such a detector for detection of high-energy radiation, its detector material also containing additional doping. Such detectors are used in X-ray computer tomography.
The object of this invention is to provide a detector arrangement which will permit a better separation of the low energy components of a polychromatic X-ray from the high-energy components, and consequently, to create a better determination of a material type of an object through which the X-rays are passed.
The idea on which the present invention is based is to obtain a better energy separation through a coordinated choice of materials of a low energy detector and a high-energy detector. In the case of the low energy detector material, according to the present invention, a material is provided which has a low self-absorption of the scintillator light generated and a spectrum of the scintillation light adapted to the spectral sensitivity of the photodiode, as well as an advantageous chemical composition in addition to having a high efficiency in conversion of X-rays into light. Therefore, the detector material of a low energy detector has elements with atomic numbers in the range of 30 to 40. The thickness of the low energy detector is selected so that the low energy X-ray quanta are absorbed almost completely, but high-energy X-ray quanta with energy of more than 50 keV, for example, mostly pass through the low energy detector. The required great homogeneity of the thickness can be achieved by surface machining, such as polishing or lapping, for example. Advantageous embodiments of these low energy detectors are resistant to moisture and temperature fluctuations. Zinc selenide doped with tellurium has these properties in particular.
The high-energy detector is preferably made of a dense material consisting of elements with large atomic numbers in the range of 56 to 83, so that this detector can be made as thin as possible. This detector material is preferably a ceramic gadolinium oxysulfide doped with at least one rare earth element.
Because of the coordinated choice of materials of the low energy detector and the high-energy detector, the detector arrangement has a low persistence and is thus definitely improved in comparison with the energy-selective detector arrangements known in the past.
In the case of a linear or flat arrangement of such detector pairs, there is the effect that scintillation light is propagated in the detector material and is detected in adjacent photodiodes. This so-called optical crosstalk leads to reduced contrast in the X-ray image and also falsifies the determination of the material. This effect increases greatly with an increase in thickness of the material of the detector and reduced self-absorption of the scintillation light. To counteract this effect, the detector is segmented into individual pixels and separated by a reflective layer of titanium oxide or aluminum oxide, for example. This separation takes place with both the low energy detector and the high-energy detector.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.